We have demonstrated complete suppression of several mouse tumors with either of 2 strains of tubercle bacilli (H37Ra, BCG) or an RNA rich fraction isolated from H37Ra. Suppression was due to an altered host response since treated tumors grew in immune suppressed mice. Now we have established a mouse model for the assay of bacterial fractions which cause regression of a growing tumor. P815 mastocytoma is inoculated into the foot pads of syngeneic DBA mice. After 6 days, when tumor growth can be demonstrated, bacterial preparations are injected either intralesionally or via other routes and the foot pads are measured periodically with a "Schnelltaster." Our most promising results are with the RNA rich fraction isolated from H37Ra, a cell wall skeleton fraction (H37RA) and heat killed C. parvum. All mycobacterial fractions appear to suppress or cause regression of growing tumor when the tumor load is limited and the treatment is intralesional. However, the RNA rich fraction still causes regression under a heavy tumor load (both foot pads given tumor and only one treated). The cell wall skeleton fraction under the same conditions, causes enhancement of tumor growth. Crude cell wall fractions lack activity in this model system. C. parvum, an effective agent in these regression studies, is more effective when injected intralesionally than intraperitoneally or subcutaneously. BIBLIOGRAPHIC REFERENCE: Millman, I., Scott, A. W., Halbher, T., Kavanaugh, M.,Maguire, H., Youmans, G.P., and Youmans, A.S. Antitumor effect of bacteria and bacterial fractions. Submitted to American Association for Cancer Research for session on Virology and Immunology, Toronto, Canada, April 30--May 9, 1976, Abstract.